Cross-linked maleic anhydride copolymers



3,002,955 f CROSS-LINKED MALEIC RIDE COPOLYMERS This invention relates to adhesives and, in preferred embodiments, to compositions which may be used as pressure-sensitive adhesives.

In manufacture of known adhesives it has been necessary to compound several ingredients in order to arrive at an acceptable formulation. In manufacture of tape, notably transparent pressure-sensitive tapes, or tapes used for surgical purposes, it has been the usual practice to compound together, for example on a heated mill equal parts of an elastomer, a tackifier and a pigment or filler. It has further been found necessary to add a substantial amount of plasticizer in order to provide the desired plastic properties and to permit the adhesive to be readily applied to the backing. An additional ingredient, nearly always found necessary, is a stabilizer, e.g. an antioxidant, which tends to impart resistance to aging conditions such as air, heat and sunlight. The compounding of these various ingredients itself has constituted a significant problem since it has been necessary to mix them on a pair of rolls which is a time-consuming and expensive operation. With so many ingredients, variations in supply and quality of raw materials have been found to give rise to serious difliculty. The elastomer and the tackifier are particularly susceptible to these circumstances. Even after obtaining the compounded adhesive, ones difficulties were not always finally resolved. It has frequently been impractical to obtain the degree of tack and tensile strength in the end product adhesive since, these variables are diflicult to control and frequently could not be controlled to the extent nor within the range desired. Migration of the plasticizer from the adhesive composition into surrounding areas or to adjacent films or layers has also been the cause of considerable difficulty.

Objects and advantages of the present invention are solution of problems which arise by reason of the abovelisted disadvantages in prior art adhesives. One object is to produce an adhesive having high stick properties which does not require the presence of a tackifier and, in particular, an adhesive which may be compounded to stick to an unprimed cellulosic film such as ethyl cellulose or cellulose acetate. A further advantage of the invention is that compounds have been discovered which may be so constituted that they will have plasticity and other properties variable over a wide range, but reproducible and controllable by the method of preparation. Another object of the invention is to provide compounds which have high tensile strength and approach water whiteness so that they may be used in pressure-sensitive commercial transparent tapes.

Still another object of the invention is to provide, according to one form of the invention, an adhesive compound which may be used, preferably without plasticizer, and will be stable, that is, resistant to aging conditions such as heat, sunlight and air without the need for a stabilizer.

The adhesive compounds of the present invention are certain esters of maleic anhydride-vinyl ether copolymers. Compounds possessing peculiarly valuable adhesive properties according to the invention are such copolymers esterified to the extent of at least 65% of the total potential carboxyl content with a primary aliphatic alcohol having 6 to 16 carbon atoms, inclusive. The invention is also directed to the method of preparing these compounds and their use in certain articles such as 1 ted States Pgtgnt 1 3,302,955. Patented Qct. 3, 1961 tapes, surgical dressings, and other articles having a supporting surface with a layer of the invention adhesive coated thereon.

An important advantage of the invention is the provision of adhesives which may be cured to an insoluble, solvent-resistant, heat-resistant material, and, still remain pressure-sensitive. In marked contrast to prior art ad hesives, which generally will stick to themselves tena ciously but to other objects with relatively little force, the invention adhesive is characterized by tenacious adhesion" to other objects such as the human skin, but relatively little adhesion to itself. Hence, adhesive tape made from invention adhesive will not stick to itself to the degree that separation is prevented without disruption of the mass or undue deformation of the tape backing.

The vinyl ether-maleic anhydride polymers of the invention are prepared by catalytic copolymerization of the monomers in a solvent, preferably a nonaqueous solvent such as xylene or benzene, and in an inert atmosphere such as nitrogen. Approximately equimolecular amounts of the monomers are first dissolved in the solvent. The solution is then heated to reaction temperature which is in the range above about 40 C. to the boiling point of the solution, and maintained at this level until the reac tion has progressed to completion. At higher temperatures, of course, the reaction proceeds at a faster rate. It has been found that the reaction rate may be best controlled at temperatures in the range 40 to C., and hence this temperature range is preferred. If desired, the reaction rate may be eifectively controlled at higher temperature levels by adding the vinyl ether and catalyst stepwise.

The molecular weight of the maleic anhydride-vinyl ether polymer is controlled by controlling primarily three variables, i.e., the temperature, the catalyst concentration and the solvent utilized for carrying out the polymerization. Higher temperatures tend to produce low molecular weight polymers and, conversely, low temperatures tend to produce higher molecular weight copolymers. A peroxide catalyst, such as benzoyl peroxide, which is soluble in the solvent medium is added to catalyze the reaction, which is a chain type reaction. There is a tendency on the part of the solvent to interfere to a degree with the polymerization, that is, to combine with the free radical which promotes the reaction, and thereby terminate the reaction at the point at which the polymerization has progressed on a particular chain. This interference on the part of the solvent has been designated by an index known as the transfer constant, a term proposed and defined by Mayo, Journal of the American Chemical Society, vol. 65, 1943, pages 2324 to 2329. Solvents having a low transfer constant exhibit relatively slight tendency to interfere with chain-reaction type polymerization. Benzene is an example of this type solvent. On the other hand, solvents having a relatively high constant tend to have high degree of interferencewith the polymerization. Xylene is an example of the latter type solvent. When a solvent having a lower transfer constant is used the molecular weight of the polymer is sensitive to the concentration of catalyst, larger amounts of catalyst tending to produce lower molecular weight polymers. The low constant solvents tend to produce higher molecular weight polymers, and thehigher constant solvents tend to produce lower molecular weight polymers. However, the use of a higher constant solvent such as xylene is not precluded where high moleculer weight polymers are desired since the tendency of the xylene to lower the molecular weight may be counteracted to some extent by operating at a lower reaction temperature.

As an example of copolymerization of maleic anhydride and vinyl ether, 1.0 mol of maleic anhydride is dis solved in 600 parts of xylene as solvent. The reaction vessel is provided with a superjacent vented reflux condenser. Heat is applied to the vessel while stirring its contents and while gaseous N is being passed through the vessel until a temperature of 65 C. is attained. At this point 1.0 mol of ethyl vinyl ether and 0.33 part of benzoyl peroxide catalyst are added and the 65 C. tem perature and stirring continued until reaction is complete. The polymer precipitates as it is formed. The mixture of polymer and solvent produced may be used for preparation of polymer ester as such by adding appropriate amounts of alcohol, antioxidant and esterification catalyst as indicated below, or the polymer and solvent may be separated by filtration prior to esterifying the polymer. Any possible small amounts of unreacted monomers will be removed from the system during puri- =fication following esterification.

Attempts to polymerize maleic ester with vinyl ether have sufiered from the tendency of the ester and ether to polymerize in ratios other than that sought for according to the present invention. Further, it is diiiicult to obtain the high molecular weights and plasticities which are important properties of the invention products. Hence, the invention products are preferably made by using the maleic anhydride as monomer, rather than an ester thereof.

The index of polymer molecular weight is the intrinsic viscosity of the polymer. This is measured by preparing a dilute solution of the polymer in cyclohexanone as solvent, and carrying out intrinsic viscosity measurement at 30 C. according to known procedure. For example, a cyclohexanone solution of the polymer having 1 gram of polymer per 100 cc. of solution is prepared, and several dilutions of this solution, e.g. to 0.7 grams per 100 cc. and 0.5 gram per 100 cc., are made. Flow times are then measured at 30 C. on a general purpose Ostwald viscosimeter pipette having a capillary about 0.5 mm. in diameter and a length of about 15 cm. The relative viscosity at each concentration is calculated. Relative viscosity is the ratio of the flow time of the resin solution to the flow time of the solvent. Specific viscosity is defined as the relative viscosity minus one. Reduced specific viscosity is defined as the specific viscosity divided by the concentration in grams per 100 ml. of solution. A plot is made of reduced specific viscosity vs.

concentration and the straight line so obtained is extrapolated to zero concentration. The value of the reduced specific viscosity at zero concentration is known as the intrinsic viscosity.

The polymer molecular weight as measured by intrinsic viscosity exerts a notable and important effect upon the properties of the final ester adhesive, as will appear more clearly below. High molecular weight polymers tend to produce adhesives having higher plasticity, and low molecular weight polymers tend to produce adhesives having low plasticity. Suitable polymers are those having viscosities within the approximate range 0.2 to 2.0 while preferred polymers have viscosities within the approximate range 0.4 to 1.7. The factor of plasticity and its effect upon properties of the adhesive is gone into more fully below whereupon the importance of controlling the polymer viscosity will be appreciated.

The vinyl ether used as monomer for preparation of the invention polymers may be any of the low molecular weight vinyl ethers, that is, those having one to eight carbon atoms, inclusive, in the aliphatic group. Methyl, ethyl, propyl, isopropyl, butyl and isobutyl vinyl ethers are readily polymerizible and hence notably satisfactory. The general formula of the ethers is:

where R is alkyl having 1-8 carbons, preferably 1-2 carbon atoms, inclusive, and X is a substituent such as low molecular weight alkyl, that does not seriously interfere with the stability, nor the polymerizability of the vinyl ether. As the ether molecular weight increases, some plasticizing effect upon the final polymer is introduced, but the higher molecular weight ethers may be found somewhat more difficult to co-polymerize with the maleic anhydride.

Both the vinyl ether and the maleic anhydride polymerize with the other monomer rather than with itself, and hence the ratio of monomers in the copolymer is one to one.

The" particular alcohol used to esterify the polymer ester has an important effect upon the properties, such as plasticity, of the final adhesive produced. The ten carbon atom alcohols produce adhesives having lowest plasticity, i.e. the softest adhesives. As alcohol molecular weight is decreased below n-decyl alcohol the tendency is to produce adhesives which are harder, i.e. have a higher plasticity. As alcohol molecular weight is increased above 10, the polymer esters again increase in plasticity. The choice of alcohol is a convenient variable which may be employed in producing an adhesive having the particular sought-for properties in any case. Generally, satisfactory adhesives may be prepared from primary monohydric aliphatic alcohols having 6 to 16 carbon atoms, inclusive. These may be either straight or branch chain alcohols. The straight chain alcohols are, however, preferred since they tend to produce softer and tackier adhesives, thereby giving more latitude in manu facture of an adhesive having specific sought-for properties. The lower carbon atom alcohols and the high carbon atom alcohols within the broad range may find particularly utility as laminating adhesives since they tend to be harder. However, as indicated above, it is an important advantage of the invention to manufacture pressure-sensitive adhesives which do not require use of plasticizers or tackifiers. Alcohols having 6 to 16 carbon atoms, inclusive, are therefore preferred for manufacture of pressure-sensitive adhesives since the degree of softness and plasticity of adhesives obtained by use of alcohols within this range is such that tackifiers and plasticizers may be dispensed with. Normal decyl alcohol, noctyl, n-hexyl and lauryl alcohols are preferred materials for manufacture of adhesives according to the invention. Another very suitable alcohol is known as CPS 224, a product of Enjay, Standard Oil Company of New Jersey, which is a mixture of alcohols containing a high proportion of branch chain decyl alcohols. Other satisfactory alcohols for manufacture of pressuresensitive adhesives are n-nonyl, iso-octyl (Enjay), tridecyl (Enjay) and n-undecyl alcohols.

The percentage of esterification of the copolymers is another factor having an important effect upon the adhesives. Percent esterification is defined as the over-all percentage of total potential carboxyl groups on the polymer chains that are in the form of the ester group --COOR, where R is the alkyl part of the alcohol. Potential carboxyl groups include carboxyl groups as such and groups capable of yielding the carboxyl group by hydrolysis and having the formula where X is hydrogen, alkyl, metal or carbon (e.g. as in the case of the anhydride). The higher the percentage of esterification, the greater the tack possessed by the adhesives and the softer the adhesive becomes. Below 65% esterification, the esters possess insufiicient tack for invention purposes. The important sought-for properties of the invention adhesives begin to become apparent at percent of esterification above 65%. Hence, the invention contemplates adhesives esterified in the range of 65 to Adhesives in the lower portion of this range possess inherent tacky properties but may be harder so as to require the addition of certain amounts of plasticizers. However, at percent of esterification above accesses about 80-85%, sufficient alcohol-will have been-incor-j porated into the adhesive to impart softness so that addition of extraneous plasticizer is not necessary, and the adhesives will be found to be self-plasticized.

The percent of esterification is also important from the standpoint of leaving enough unreacted carboxyl groups to permit cross-linking of the polymer chain by reinforcing action between unesterified carboxyls, as will be more fully explained below. Below about 95% esterification it will generally be found that there are sufficient unreacted, i.e. unesterified, carboxyl groups to permit desired cross-linking of polymer chains by reinforcing action. Hence, in one embodiment of the invention a range of esterification of particular importance is approximately 8085 to 95%. As previously indicated, higher percentage of esterification tends to produce adhesives which are softer. However, even at esterifications approaching 1.00% the plasticity of the adhesive can be controlled by choice of molecular weight of alcohol and degree of polymerization of maleic anhydride-vinyl ether copolymer. Further, the higher the esterification of the copolymer, the less is its tendency to cross-link during heating, aging, etc., and hence the greater its stability. Adhesives approaching esterification of 100% have the outstanding advantage of optimum degree of tack. Their softness and tendency to cold flow can be controlled by methods explained below.

Reaction conditions for esterification of the polymer are important in obtaining the high percentages of esterification required according to the present invention.

The esterification reaction of the copolymer is preferably, though not necessarily, carried out in the same solvent that was used for polymerization. However,'the polymer raw material may be purchased, and dissolved in solvent rather than synthesized. A molar excess of the desired alcohol is added to the solution together with an anti-oxidant and catalyst. Suitable anti-oxidants include 2,5-ditertiarybutylhydroquinone, 2,6-ditertiary butyl p-cresol, and others known in the art, while as catalysts p-toluene sulfonic acid, benzene sulfonic acid, sulfuric acid and many others are satisfactory. The solution is then boiled and water of esterification removed until reaction is substantially complete. The completeness of the reaction is observed by the rate of removal of water and when the rate becomes insignificant the reaction is considered as being substantially complete. Hydrocarbons, preferably aromatic hydrocarbons, boiling above 100 C. are suitable solvents. Using diethyl benzene as a solvent (boiling point 182 C.) completion of the reaction may occur in about 70 minutes. With xylene (B.P. 139-144 C.) the reaction may take 7 to 8 hours, while with toluene (BJP. 110.6 C.) 12 or more hours may be required for substantially complete esterification. The concentration of esterification catalyst has considerable effect upon the rate of reaction. After the esterification reaction has progressed to the desired degree the reaction is terminated by cooling the reaction mass.

Various procedures may be used for recovering the adhesive from the solution. For example, the reaction mass after esterification may be placed in a vessel fitted with a mechanical agitator and an outlet at the bottom. Methanol may then be added, with stirring, until the ester just precipitates sufficiently to form two distinct layers upon being allowed to settle. The ester is drawn off from the bottom of the flask. The methanol layer, containing impurities, may be treated to recover solvents, as by distillation. The ester is put back into the vessel and just enough toluene or benzene added to redissolve it. Again, methanol is added until the ester just precipitates. It is allowed to settle, and again separated from the methanol layer. This procedure is repeated twice more. At this point the ester will have acquired a very light yellow color. Generally a total amount of methanol equal to only one-half the volume of original ester solution is required for purification.

An alternative procedure for recovering adhesive which is -95% or more esterified is to pour-the reaction mass after cooling into a sufficient amount of acetone to cause precipitation of the polymer. The liquid is decanted and the polymer is washed with small amounts of acetone. The polymer at this stage will be almost water-white.

The polymer solution after purification may be treated with a small amount, i.e. 2%, of an anti-oxidant stabilizer such as 2,5-ditertiarybutylhydroquinone to stabilize the polymer during drying. The solution is then evaporated on a steam bath and the semi-dried polymer is dried in a vacuum chamber at 100 C. and 15 inches of mercury absolute pressure.

Polymer ester plasticity is a key property determining its suitability for use in various types of adhesives. The plasticity should be high enough to afford adequately high tensile strength and low enough so that it can be successfully worked and calendered or spread onto the sheet. Plasticity is measured on a cylindrical pellet of adhesive mass 16 mm. in diameter and weighing 2.00 grams by placing the pellet on a parallel plate plastometer, otherwise known as a Williams plastometer, of the type manufactured, for example, by the Scott Instrument Company, and maintaining the pellet and surroundings at 100 F. and under a 5000 gram load, and noting the height of the pellet after 14 minutes. This height in millimeters is taken as a measurement of plasticity. For invention purposes, polymer esters having plasticity so low as to be outside the range of accurate measurement on the instrument may be satisfactory. Methods are available to increase plasticity, as will be explained below. The desired plasticity is obtained, as implied above, by controlling viscosity of the copolymer (high viscosities tend to produce high plasticities), controlling the alcohol used for esterification (alcohols having very high or very low molecular weight tend to produce adhesive having high plasticities), and controlling the degree of esterification (high esterification tends to produce low plasticity). On the other hand, adhesives which have higher plasticities, that is, above the range of those which may be used unplasticized as pressure-sensitive adhesives may be used as laminating adhesives or, by incorporating suitable amounts of plasticizer, as pressure-sensitive adhesives. However, adhesives in the plasticity range substantially above 4.0 mm. are less advantageous when pasticized for use as pressure-sensitive adhesives since they may lack the necessary cohesive strength. It is generally endeavored to produce adhesives having plasticities not substantially above 4.0 mm. The preferred range of plasticity is in the approximate region 0.7-3.0 mm, since within this range polymer esters have plasticity such that no plasticizer is necessary to produce ad hesive masses having a high degree of tack and pressuresensitivity.

As indicated above, there is a tendency on the part of certain of the invention adhesives to exhibit some cold flow, that is, lack of body, and suspectibility to plastic flow over long periods of time when under stress. This phenomenon may be overcome by cross-linking the polymer ester chains. Cross-linking may be obtained by formation of reinforcing bonds between unreacted carboxyl groups on adjacent polymer chains. This type of cross-linking may be produced by a polyvalent metal oxide type filler (including hydrates) such as zinc oxide, titanium dioxide or aluminum hydrate. The metallic oxide compound in any case may be dispersed in the adhesives in a ball mill or paint mill, or can be mixed on the rolls. The controlling factor in formation of reinforcing bonds is the percentage free, i.e. unesterified, carboxyl groups in the polymer ester rather than the amount of metal oxides present since very small amounts of metal oxide will accomplish a high degree of crosslinking. In the case where cross-linking and gelling of the adhesive ester is relied upon to impart resistance to cold flow, the percent of esterification as indicated above, is of importance. The range 80-85 to esterification defines the content of free carboxyls which will atford adequate cross-linking. Above about 95% esterification the polymer esters are relatively immune to cross-linking by formation of reinforcing bonds.

Another convenient way of imparting resistance to cold flow to a polymer ester is to incorporate an adequate amount of a copolymer ester adhesive which has previously been cross-linked, either by the reinforcing bond formation as previously described or by peroxide reaction, described below, since such gelled materials are quite compatible with the adhesive.

The alternative and preferred manner of effecting polymer cross-linking is to carry out a chain type reac tion by addition of a free radical type polymerization catalyst, e.g. a peroxide catalyst such as benzoyl peroxide. This is accomplished by adding to the purified Solution of adhesive ester in a solvent such as toluene, an amount of the benzoyl peroxide cross-linking catalyst in the range 0.1 to 10% based on the ester, heating the solution to remove solvent at about 100 (3., heating the residue for time to accomplish cross-linking which will generally be in the range to 60 minutes, and then terminating the cross-linking reaction by heating until all peroxide has been decomposed, e.g., overnight, at 100 C. in the case of benzoyl peroxide.

An index of the degree of cross-linking is gel content, which may be determined as follows, 0.4 gm. of adhesive is mixed with 100 cc. of benzene and allowed to stand without stirring for 48 hours at 30 C. The mixture is then poured through a standard stainless steel wire screen of 100 mesh. 25 cc. of the filtrate are evaporated to dryness at 100 C. and the residue weighed. The gel content is taken as 100 minus the percentage of the original sample of adhesive which dissolved. High gel content, of course, reflects high degree of crosslinkage. Further, the degree of cross-linking required in order to reduce cold flow to the sought-for levels specified below will be determined to a large extent by the plasticity of the ester, low plasticity polymer esters requiring generally more cross-linking than high plasticity esters. From the standpoint of sufiicient cross-linking to reduce cold how to acceptable levels, even for the higher plasticity esters, minimum gel content of about is desirable for material cross-linked either in the first instance or by addition of previously cros'sdinked ester. Where consistent with maximum plasticity limitations, gel content of invention products may be as high 'a's 100%.

If the occasion requires, cross-linking may be efiected by both metal oxide and peroxide action. In place of a peroxide any of the cross-linking agents employed as rubber-curing agents, i.e. those which operate by a free radical mechanism, may be utilized.

The property of cold flow may be measured by sticking a ball of the adhesive weighing 3.0 gms. against a vertical glass plate and noting the downward flow in inches due to gravity for a period of 24 hours at room temperature. Particularly suitable pressure-sensitive adhesives of the invention are those which are cross-linked to the extent that cold flow is not greater than 2 /2 inches as measured by this standard method.

Among the outstanding advantages of the invention products is their ability to stick to unprimed film, e.g. 'cellulosic films such as ethyl cellulose, cellulose acetate, or (when cross-linked) to regenerated celluose. The adhesive compounds further have the property of sticking to unprimed films of polyvinyl chloride resin, and polyethylene glycol terephthalic acid film which is marketed by E. I. du Pont de Nemours & Company under the trademark Mylar.

The advantages of the invention composition are especially outstanding in respect to adhesiveness, particularly pressure sensitivity. However, the novel compositions of the invention possess other advantageous properties which render them suitable for other applications. For example,

by cross-linking the products, one may obtain a bonding material which is insoluble in solvents and resistant to elevated temperature, and hence is suitable for manufacture of plywood or glass fiber laminates. Certain of the invention products may be incorporated in coatings as curing resins. They possess unusually strong adhesion to glass and metal, good gloss, and can be cured by baking with a free radical type catalyst. At temperatures below that of pressure sensitivity the invention products are also applicable to injection molding where curing is desired.

The following examples are presented as illustrating rather than limiting the scope of the invention. Parts and percentages are on a weight basis unless otherwise specified.

Example I An esterification vessel was fitted with an azeotropic trap and stirrer and the following materials were placed therein: 39 parts vinyl methyl ether-maleic anhydride copolymer having intrinsic viscosity 1.66, 102 parts lauryl alcohol, 5 parts p-toluene sulfonic acid, 8 parts 2,5-ditertiarybutylhydroquinone and 344 parts xylene. The mixture was heated to boiling whereupon the copolymer and other ingredients went into solution. Refiuxing was continued for 3 hours during which time 4.5 parts of water were collected in the azeotropic trap. The esterification reaction was substantially complete as evidenced by decrease in rate of removal of water of esterification. The solution was cooled and poured into just suflicient methanol to cause precipitation. After phase separation, the precipitate was treated with just enough benzene to dissolve it. The polymer ester was again precipitated and dissolved as above, and reprecipitated and washed several times with small amounts of methanol. The wet precipitate was dried on a steam bath and finally more thoroughly dried at C. and 15 in. mercury absolute pressure. The product was extremely tacky, almost water-white and had plasticity of 1.30 mm. Percent esterification was about 94%. Adhesive masses for use as surgical adhesives may be made by milling the polymer ester with 40-50% of zinc oxide, titanium dioxide or aluminum hydrate, etc. Alternatively, the masses may be cross-linked according to the peroxide procedure described above and exemplified in Example XI. These masses are spread on primed vinyl film, cloth or waterproof cloth.

Example 11 In an esterification apparatus similar to that of Example I there were placed 78 parts vinyl methyl ethermaleic anhydride copolymer having intrinsic viscosity 0.73, 204 parts lauryl alcohol, 5 parts p-toluene sulfonic acid, 8 parts 2,5-ditertiarybutylhydroquinone and 344 parts xylene. The mixture was refluxed for 3 /2 hours during which time 9.8 parts water were collected in the azeotropic trap. The polymer ester was recovered and purified according to the procedure described in Example I. It was extremely tacky, almost water-white and had plasticity of 0.73 mm. Percent esterification was 86%.

Example Ill 39 parts of vinyl methyl ether-maleic anhydride copolyrner having intrinsic viscosity of 0.73 were placed in an esterification apparatus as described in Example I together with 61 parts n-hexyl alcohol, 4 parts p-tolucnc sulfonic acid, 7 parts 2,5-ditertiarybutylhydroquinone, and U2 parts xylene. The mixture was refluxed for 5 /2 hours after which 6.4 parts water had been collected, and the reaction was considered to be substantially complete as evidenced by rate of water liberation. The reaction product was recovered and purified as in Example I. The dried polymer ester had plasticity of 1.05 and was extremely tacky. Percent esterification was 96%.

Example IV In the esterification apparatus described in Example I there were placed 156 parts vinyl methyl ether-m'al'eic anhydride copolymer, intrinsic viscosity 0.73, 312 parts Example V Into the esterification apparatus of Example I there were placed 39 parts of vinyl methyl ether-maleic anhydride copolymer having intrinsic viscosity 0.73, 95 parts n-decyl alcohol, 4 parts p-toluene sulfonic acid, 7 parts 2,5-ditertiarybutylhydroquinone and 172 parts xylene. The mixture was refluxed for 5 hours, and 6.2 parts water were collected in the azeotropic trap. The polymer ester, after purifying and drying, was found to be extremely tacky and have plasticity less than 0.65. The. percent esterification was 94%.

; Example VI In the esterification apparatus described in Example I there were placed 39 parts vinyl methyl ether-maleic anhydride copolymer having intrinsic viscosity 0.73, 95 parts of a mixture of alcohols containing a high proportion of branch chain decyl alcohols known in the trade as CPS 224, a product of Enjay, Standard Oil Company of New Jersey, 4 parts p-toluene sulfonic acid, 7 parts 2,5- ditertiarybutylhydroquinone and 172 parts xylene. The mixture was refluxed for 9 /2 hours, and 7.1 parts water were collected. The polymer ester, after separation, purification and drying, was found to be extremely tacky and have plasticity less than 0.65. Percent esterification was 99%.

Example VII 39 parts vinyl methyl ether-maleic anhydride copolymer having intrinsic viscosity 0.73, 78 parts iso-octyl alcohol, 3.5 parts p-toluene sulfonic acid, 6.5 parts 2,5- ditertiarybutylhydroquinone, and 172 parts xylene were placed in an esterification apparatus and refluxed for 9 hours. 8.3 parts water were collected in the azeotropic trap. The polymer ester, after recovery, purification and drying, was extremely tacky, and had plasticity of 1.14. Yield was 87% of theory. Percent esterification was 9.6%.

Example VIII In an esterification apparatus fitted with an azeotropic trap and a mechanical stirrer were placed 156 parts maleic anhydride-vinyl methyl ether copolymer, intrinsic viscosity 0.73, 312 parts n-octanol, 14.5 parts p-toluene sulfonic acid, 22.5 parts 2,5-ditertiarylbutylhydroquinone, and 515 parts xylene. The mixture was heated to boiling, refluxed, and water was removed in the azeotropic trap. 7 samples were removed from the esterification vessel at various times during the reaction. Percent esterification was calculated for each sample from the amount of water collected at the time of removal. The first two samples were precipitated and purified with petroleum ether and the remaining samples were precipitated and purified with methanol. Several redissolvings and reprecipitations were carried out on each sample. Plasticities and appearances of the samples were as follows:

Example IX In a polymerization vessel fitted with a reflux condenser, an electric heater, a mechanical stirrer and a thermometer were placed 29 parts of maleic anhydride and 430 parts of benzene. The mixture was heated to 70 C.- Nitrogen was passed through the vessel continuously.- The maleic anhydride went into solution. 26 parts of vinyl ethyl ether, which had previously been purified by multiple extraction with 5% aqueous sodium carbonate, in which ether there was dissolved 0.25 part of benzoyl peroxide, were added to the benzene solution. Polymerization commenced after 5 minutes heating as evidenced by a clouding of the solution. After 3 hours at 70 C. the reaction was substantially complete. The $118. pension was filtered, and the filter cake resuspended in benzene and filtered again. Yield was and intrinsic viscosity 0.5. In an esterification vessel similar to that employed in Example I there were placed 43 parts of the above copolymer, 61 parts n-hcxyl alcohol, 3 parts p-toluene sulfonic acid, 5 parts 2,5-dite1tiarylbutylhydroquinone, and 172 parts xylene. The mixture was heated to boiling and refluxed for 8 hours while collecting 5.5 parts of water. The esterification reaction was considered to be substantially complete as evidenced by low. rate of water removal. The polymer ester was recovered, purified and dried according to the procedure in Example I. The ester was extremely tacky, almost water-white, and had plasticity of about 0.7. Percent esterification was 93%.

. Example X Into the polymerization vessel of Example IX there were placed 48.5 parts maleic anhydride and 300 parts benzene. The mixture was heated to 70 C., passing nitrogen through the vessel continuously. 50 parts vinyl-n-butyl ether which had been washed with aqueous sodium carbonate and which contained 0.5 part benzoyl peroxide were added to the benzene solution. Polymerization commenced after 1 minute. After 3 hours of stirring at 70 C. the reaction was substantially complete. The polymer was recovered and had intrinsic viscosity of 0.68. Yield was 92% of theory. 87 parts of the copolymer ester were esterified with 168 parts n-decanol in the presence of 7.5 parts p-toluene sulfonic acid, 12.8 parts 2,5-ditertiarybutylhydroquinone, and 258 parts xylene. After 7 hours of refluxing 10.2 parts water had been collected in the azeotropic trap. The polymer ester was recovered, purified and dried. It had good tack and plasticity less than 0.65. Percent esterification was 91%.

Example XI tion was divided into 5 portions. To the 5 portions there were added 5%, 1%, 4%, and 0%, respectively, benzoyl peroxide based on the weight of polymer ester in the solution. The solutions were then heated to remove solvent and the residues were heated for 3 hours at C. Measurements of gel content, plasticity and cold flow were made on the several samples, and results of the tests are tabulated below:

Percent Percent 001d Portion N o. Peroxide Gel Plasticity Flow, Inches 5 94. a 2. 75 0 1 78.0 1.63 y 36 72.0 0.99 s2 54 42.0 0.70 7% 0 00.0 0.65 8% Y Any of the adhesives made according to any of the foregoing examples may be utilized by applying it to an adhesive-supporting surface, plasticized where necessary, or it may be cross-linked to impart resistance to cold flow and applied to an adhesive supporting surface. 7

Although centain of the foregoing examples specify the use of an anti-oxidant during esterification, under some conditions an anti-oxidant is not required. Specifidaily, when the viscosity of the copolymer is less than one, anti-oxidant may be dispensed with without incurring undesirable degree of gelling, i.e. cross-linking during esterification. The following example illustrates preparation of polymer ester according to the invention without employing anti-oxidant.

Example XII 467 parts of vinyl methyl ether-maleic anhydride copolymer having intrinsic viscosity 0.73 were esterified according to the procedure described in the foregoing examples with 937 parts iso-octyl alcohol in the presence of 44 parts p-toluene sulfonic acid catalyst and 1600 parts xylene *as solvent, without anti-oxidant. Refiu'xing was carried on for 8 hours during which time '102 parts of water were collected in the azeotropic trap, and esterification had progressed to 9798% completion. The e'sterification solution was light in color with a slightly yellowish cast. The polymer ester was recovered from solution, washed and dried. It showed no tendency to gel upon cooling and was very tacky. This adhesive was suitable for applying to an adhesive-supporting surface, or cross-linking to impart resistance to cold flow.

A remarkable feature of the invention is the capacity of the cross-linked polymer esters to stick very tightly to what are generally considered to be stick-resistant surfaces such as glass. A particularly notable property of the invention materials, when cross-linked, is their ability to stick very tightly even to polyethylene. On the other hand, the uncross-iinked polymer esters are notable for their tendency to stick only to skin, while having minimum adhesion to such other objects and minimum adhesion to themselves.

Another group of cross-linking agents are the diand tri-hydroxy alcohols such as the glycols (e.g. ethylene and propylene glycols and condensation products thereof such as diethylene and dipropylene glycols, etc.) and glycerol. For example, a benzene solution of the iso-octyl vinyl methyl ether-maleic anhydride copolymer ester prepared according to Example II, uncross-linked, 97% esteriiied, plasticity 0.81 mm., is mixed with diethylene glycol in weight amount equal to the copolymer ester (a substantial molar excess bases on unesterified carboxyl groups) and 2% p-toluene sulfonic acid based on polymer ester, in a beaker and heated with stirring to 140 C. as benzene evaporates. Heating is continued for /2 hour. The beaker contents are then poured into methanol. The cross-linked material precipitates and is reprecipitated several times from benzene with methanol to purify. A convenient variation is to add the polyhydric alcohol directly to the copolymer esterification reaction mixture anl continue heating until the desired amount of crosslinking occurs (about 2 hours) as just described. Plasticity is 2.1 mm. Cold flow is 0.4 inch.

It will be apparent that other modifications may be made within the scope of the invention. For instance, substituted vinyl ethers, such as methyl vinyl ethers, when 12 reasonably stable, polymerizable and available, may be utilized instead of the monomers described specifically above. The invention is not limited to the illustrations presented.

This application is a division of application Serial Number 380,595, filed September 16, 1953, which issued as U.S. Patent No. 2,866,775 on December 30, 1958.

The claims are:

1. An adhesive composition having a plasticity not greater than 4.0 mm. and a cold flow not greater than 2.5 inches, comprising a cross-linked ester of a copolymer comprising about equimolecular proportions of maleic anhydride and an alkyl vinyl ether the alkyl group of which contains from 1 to 8 carbon atoms, said copolymer having an intrinsic viscosity in the range from about 0.2 to 2.0, about 65 to percent of the potential carboxy groups in said copolymer being esterified with an aliphatic primary monohydric saturated alcohol containing 6 to 16 carbon atoms, said ester being cross-linked with an inorganic polyvalent metal oxide to impart to said ester a gel content in the range from about 10 to percent by weight of the ester.

2. A method for the preparation of a composition of claim 1 which comprises forming a solution in a nonaqueous solvent of a copolymer comprising about equimolecular proportions of maleic anhydride and an alkyl vinyl other the alkyl groups of which contain from 1 to 8 carbon atoms, said copolymer having an intrinsic viscosity in the range from about 0.2 to 2.0, incorporating in said solution an aliphatic primary monohydric saturated alcohol containing 6 to 16 carbon atoms together with an esterification catalyst and an antioxidant, and heating the resulting solution to esterification temperature with removal of water of esterification from the system until at least 65 percent of the potential carboxy groups of said copolymer are esterified, adding thereto with milling a polyvalent metal oxide in an amount suflicient to impart to said ester a gel content in the range from about 10 to 100 percent by weight of the ester.

3. An adhesive product comprising an adhesive-supporting surface coated with an adhesive composition of claim 1.

4. A composition of claim 1 in which the alkyl vinyl ether is methyl vinyl ether and the primary monohydric alcohol is n-dodecauol.

5. A composition of claim 1 in which the metal oxide is zinc oxide.

6. A composition of claim 1 in which the metal oxide is zinc oxide, the alkyl vinyl ether is methyl vinyl ether and the primary monohydric alcohol is n-dodecanol.

7. A method of claim 2 in which the metal oxide is zinc oxide.

8. A product of claim 3 in which the metal oxide is zinc oxide.

' References Cited in the file of this patent UNITED STATES PATENTS 2,626,248 Brown Jan. 20, 1953 2,726,230 Carlson Dec. 6, 1955 OTHER REFERENCES Seymour et al.: Ind. & Eng. Chem, vol. 41, No. 7, pp. 1509-1513 (July 1949). 

1. AN ADHESIVE COMPOSITION HAVING A PLASTICITY NOT GREATER THAN 4.0MM. AND A COLD FLOW NOT GREATER THAN 2.5 INCHES, COMPRISING A CROSS-LINKED ESTER OF A COPOLYMER COMPRISING ABOUT EQUIMOLECULAR PROPORTIONS OF MALEIC ANHYDRIDE AND AN ALKY VINYL ETHER THE ALKYL GROUP OF WHICH CONTAINS FROM 1 TO 8 CARBON ATOMS, SAID COPOLYMER HAVING AN INTRINSIC VISCOSITY IN THE RANGE FROM ABOUT 0.2 TO 2.0, ABOUT 65 TO 95 PERCENT OF THE POTENTIAL CARBOXY GROUPS IN SAID COPOLYMER BEING ESTERIFIED WITH AN ALIPHATIC PRIMARY MONOHYDRIC SATURATED ALCOHOL CONTAINING 6 TO 16 CARBON ATOMS, SAID ESTER BEING CROSS-LINKED WITH AN INORGANIC POLYVALENT METAL OXIDE TO IMPART TO SAID ESTER A GEL CONTENT IN THE RANGE FROM ABOUT 10 TO 100 PERCENT BY WEIGHT OF THE ESTER. 